Space-time decoder and methods for decoding alamouti-encoded signals in high-doppler environments

ABSTRACT

Embodiments of a space-time decoder and methods for decoding Alamouti-encoded signals in high-Doppler environments are generally described herein. Other embodiments may be described and claimed. In some embodiments, soft-symbol outputs are generated from received symbols, a channel rate-of-change, and channel coefficients. Maximum-likelihood decoding may be performed to generate hard-symbol outputs from the soft-symbol outputs.

TECHNICAL FIELD

Some embodiments of the present invention pertain to wirelesscommunication systems. Some embodiments of the present invention pertainto decoding Alamouti-encoded signals transmitted by more than oneantenna in high-Doppler environments.

BACKGROUND

Some conventional transmitters transmit specially encoded signals usingtwo or more antennas to improve the ability of a receiver to processthese signals. For example, in some multiple-input multiple-output(MIMO) systems, Alamouti-encoded signals generated by a space-timeencoder are transmitted to help increase decoding gain and reducebit-error-rate (BER) at the receiver. In mobile environments, thetransmitter and/or the receiver may be moving, causing the receivedsymbols to be distorted by Doppler-shift. This distortion may reduce thereceiver's decoding gain and may significantly degrade the receiver'sBER, particularly at high signal-to-noise (SNR) levels.

Thus, there are general needs for receivers and methods for decodingreceived signals that may be distorted due to Doppler-shift. What arealso needed are space-time decoders and methods that increase decodinggain in environments with high-Doppler shift.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a wireless communication system in accordance withsome embodiments of the present invention;

FIG. 2 is a block diagram of a space-time decoder in accordance withsome embodiments of the present invention; and

FIG. 3 is a flow chart of a procedure for decoding signals in a channelwith high-Doppler shift in accordance with some embodiments of thepresent invention.

DETAILED DESCRIPTION

The following description and the drawings sufficiently illustratespecific embodiments of the invention to enable those skilled in the artto practice them. Other embodiments may incorporate structural, logical,electrical, process, and other changes. Examples merely typify possiblevariations. Portions and features of some embodiments may be includedin, or substituted for, those of other embodiments. Embodiments of theinvention set forth in the claims encompass all available equivalents ofthose claims. Embodiments of the invention may be referred to herein,individually or collectively, by the term “invention” merely forconvenience and without intending to limit the scope of this applicationto any single invention or inventive concept if more than one is in factdisclosed.

FIG. 1 illustrates a wireless communication system in accordance withsome embodiments of the present invention. Wireless communication system100 may include transmitter 102 and receiver 106. Transmitter 102 maytransmit radio-frequency (RF) signals using two or more antennas 112through channel 104 for receipt by receiver 106. Receiver 106 may useone or more antennas 116 to receive RF signals from transmitter 102through channel 104.

In accordance with some MIMO embodiments, transmitter 102 may transmitencoded pairs of symbols (α₀, α₁) using two antennas 112. In someembodiments, transmitter 102 may encode symbols (α₀, α₁) in accordancewith the following transmission matrix:

$\begin{matrix}{{G( {\alpha_{0},\alpha_{1}} )} = \begin{pmatrix}\alpha_{0} & \alpha_{1} \\{- \alpha_{1}^{*}} & \alpha_{0}^{*}\end{pmatrix}} & (1)\end{matrix}$

which may correspond to an Alamouti-encoded transmission of symbols (α₀,α₁). Symbols α₀, α₁ may comprise complex numbers that denote theamplitude and phase of input bits that were either amplitude or phasemodulated at transmitter 102. As illustrated by this transmissionmatrix, at a first instant in time, a first transmit antenna maytransmit symbol α₀ while a second transmit antenna may transmit symbolα₁. At a second instant in time, the first transmit antenna may transmitsymbol −α₁* while the second transmit antenna may transmit symbol α₀*,in which * refers to a complex conjugate. These encoded symbols may begenerated by a space-time block encoder within transmitter 102. Thistechnique transmits multiple copies of a data stream using two or moreantennas 112 to exploit the various received versions of the data toimprove reliability of data transfer. This may result, for example, inan increase in decoding gain and/or a lower bit-error-rate (BER) atreceiver 106.

In embodiments that use two transmit antennas 112 and one receiveantenna 116, channel 104 may comprise two channels (shown as channels113A and 113B) between transmitter 102 and receiver 106. Each channelmay have different channel characteristics, which may be represented bychannel coefficients (h₀, h₁) which describe the channel transferfunctions of channel 104. As illustrated in channel 104, first channel113A having channel coefficient h₀ may affect symbols −s₁* and s₀transmitted by a first of transmit antennas 112, and second channel 113Bhaving channel coefficient h₁ may affect symbols s₀* and s₁ transmittedby a second of transmit antennas 112. Transmitted symbols s₀ and s₁ maycorrespond to symbols α₀ and α1 respectively of the transmission matrixillustrated above. As further illustrated, signals from channel 113A and113B combine in channel 104 and are affected by noise, illustrated as n₀and n₁, resulting in signals that may be received by receiver 106,illustrated as received symbols r₀, r₁. Received symbols r₀ and r₁ maycomprise complex numbers or values that are a result of the integrationof the signals received by receiver 106. Received symbols r₀ and r₁ areactually generated within receiver 106 from the RF signals receivedthrough antenna 116, but are shown for illustrative purposes in FIG. 1within channel 104.

In a situation with no Doppler shift (i.e., a constant channel), thereceived symbols r₀ and r₁ may be represented by the followingequations:

R ₀ =h ₀α₀ +h ₁α₁ +n ₀

R ₁ =h ₀α₁ *+h ₁α₀ *+n ₁  (2)

In these equations, R₀ and R₁ correspond respectively to receivedsymbols r₀ and r₁ at first and second instances in time, α₀ and α₁represent the transmitted signals of the transmission matrix, h₀ and h₁represent the channel coefficients for channels 113A and 113Brespectively, and n₀ and n₁ represent average-white Gaussian noise(AWGN) components at the first and second instances in time. In aconventional receiver, a decoder may estimate the transmitted symbols α₀and α₁ in accordance with following equations, without taking noise intoaccount:

β₀ =h ₀ *R ₀ +h ₁ R ₁*=(|h ₀|² +|h ₁|²)·α₀

β₁ =h ₁ *R ₀ −h ₀ R ₁*=(|h ₀|² +|h ₁|²)·α₁  (3)

In these equations, β₀ and β₁ represent output decisions from aconventional decoder. In a high-Doppler environment where either thetransmitter or the receiver is moving, the received symbols may bedistorted due to the time-varying nature of channel 104. High-Dopplershift may result from situations in which transmitter 102 and/orreceiver 106 are moving in a vehicle, such as a car or train (e.g.,moving less than 300 kilometers per hour). This distortion may berepresented by the following equations:

R ₀ =h ₀α₀ +h ₁α₁ +n ₀

R ₁ =−h ₀(1+δ₀)α₁ *+h ₁(1+δ₁*)α₀ *+n ₁  (4)

In these equations, δ₀ represents the channel rate-of-change for channel113A and δ₁ represents the channel rate-of-change for channel 113B. Insome embodiments, a channel estimator within receiver 106 may calculatechannel rate-of-change δ₀ and δ₁ from channel coefficients that aremeasured at different times. These embodiments are described in moredetail below.

In a conventional receiver, the output of an Alamouti decoder may berepresented as:

β₀ =h ₀ *R ₀ +h ₁ R ₁ *=h ₀*(h ₀α₀ −h ₁α₁δ₀*)+h ₁ h ₁ *x ₀(1+δ₁*)

β₁ =h ₁ *R ₀ −h ₀ R ₁ *=h ₁*(h ₁α₁ −h ₀α₀δ₁*)+h ₀ h ₀ *x ₁(1+δ₀*)  (5)

These values may also be represented by the following equations:

β₀ =h ₀ *R ₀ +h ₁ R ₁*=(|h ₀|² +|h ₁|²)·α₀ +h ₁ h ₁*α₀δ₁ −h ₁ h ₀ *α₁δ₁*

β₁ =h ₁ *R ₀ +h ₁ R ₁*=(|h ₀|² +|h ₁|²)·α₁ +h ₀ h ₀*α₁δ₀ −h ₀ h ₁ *α₀δ₀*  (6)

These equations illustrate that β₀ and β₁ are distorted due to thechannel rate-of-change δ₀ and δ₁ in comparison with Alamouti decoderoutput in a constant channel case described previously in Equations (3).

In accordance with some embodiments of the present invention, receiver106 may include a space-time decoder that may compensate for thedistortion of a time-varying channel due to high-Doppler shift. In theseembodiments, receiver 106 generates soft-symbol outputs ν₀ and ν₁ fromthe received symbols r₀ and r₁, the channel rate-of-change δ₀ and δ₁ andthe channel coefficients h₀ and h₁. These embodiments are discussed inmore detail. In some embodiments, the space-time decoder may performmaximum-likelihood decoding to generate hard-symbol outputs x₀ and x₁from the soft-symbol outputs ν₀ and ν1. In some embodiments, hard-symboloutputs x₀ and x₁ may be generated from the soft-symbol outputs ν₀ andν₁ by detecting points in the signal constellation with the minimalEuclidian distance to the soft-symbol outputs ν₀ and ν1. Theseembodiments are also discussed in more detail below.

In some embodiments, receiver 106 may also generate revised soft-symboloutputs θ₀ and θ₁ from the initial hard-symbol outputs x₀ and x₁, thereceived symbols r₀, r₁, the channel rate-of-change δ₀ and δ₁ and thechannel coefficients h₀, h₁. In these embodiments, final hard-symboloutputs x₀ and x₁ may be generated by performing maximum-likelihooddecoding on the revised soft-symbol outputs θ₀ and θ₁. These embodimentsare also discussed in more detail below. The channel rate-of-change δ₀and δ₁ may be used to compensate, at least in part, for distortion inchannel 104 caused by high-Doppler shift. In these embodiments, thefinal hard-symbol outputs x₀ and x₁ generated by maximum-likelihooddecoding may be estimates of Alamouti-encoded symbols α₀ and α₁transmitted by transmitter 102. In some embodiments, the generation ofrevised soft-symbol outputs θ₀ and θ₁ and interim hard-symbol outputsmay be iteratively performed to generate the final hard-symbol outputsx₀ and x₁, although the scope of the invention is not limited in thisrespect.

Soft-symbol outputs ν₀ and ν₁ (as well as revised soft-symbol outputs θ₀and θ₁) may represent points in the complex plane corresponding totransmitted symbols α₀ and α₁, respectively. Hard-symbol outputs x₀ andx₁ may represent points of the signal constellation with the minimalEuclidian distance to the corresponding soft-symbol outputs ν₀ and ν1.When transmitter 102 uses binary phase-shift keying (BPSK) modulation,the further demodulation may not be necessary because hard-symboloutputs x₀ and x₁ may comprise the hard-bit outputs, and soft-symboloutputs ν₀ and ν₁ may comprise the soft-bit outputs. In some non-BPSKembodiments, hard-symbol outputs x₀ and x₁ may be further demodulatedwithin receiver 106 to generate hard-bit outputs, and soft-symboloutputs ν₀ and ν₁ may be further demodulated within receiver 106 togenerate soft-bit outputs. In these non-BPSK embodiments, transmitter102 may modulate symbols with higher modulation levels, such asquadrature phase-shift keying (QPSK) and 8-PSK, or quadrature amplitudemodulation (QAM), such as 16-QAM or 64-QAM, although the scope of theinvention is not limited in this respect.

FIG. 2 is a block diagram of a space-time decoder in accordance withsome embodiments of the present invention. Space-time decoder 200 may besuitable for use in receiver 106 (FIG. 1) and may compensate fordistortion of a time-varying channel due to high-Doppler shift.

In some embodiments, space-time decoder 200 comprises combiner 206 togenerate soft-symbol outputs (ν₀, ν₁) 207 from received symbols (r₀, r₁)201, channel rate-of-change (δ₀, δ₁) 205 and channel coefficients (h₀,h₁) 203. Space-time decoder 200 may also comprise maximum-likelihooddetector 208 to perform maximum-likelihood decoding to generatehard-symbol outputs (x₀, x₁) 213 from soft-symbol outputs (ν₀, ν₁) 207.

In some embodiments, hard-symbol outputs (x₀, x₁) 215 may be initialhard-symbol outputs. In these embodiments, space-time decoder 200 mayfurther comprise corrector 210 for generating revised soft-symboloutputs (θ₀, θ₁) 211 from initial hard-symbol outputs (x₀, x₁) 215,received symbols (r₀, r₁) 201, channel rate-of-change (δ₀, δ₁) 205 andchannel coefficients (h₀, h₁) 203. In these embodiments,maximum-likelihood detector 208 may generate final hard-symbol outputs(x₀, x₁) 213 by performing maximum-likelihood decoding on revisedsoft-symbol outputs (θ₀, θ₁) 211.

In these embodiments, received symbols (r₀, r₁) 201 may compriseAlamouti-encoded symbols (e.g., symbol pairs s₀, s₁) transmitted by twoor more transmit antennas, such as transmit antennas 112 (FIG. 1).Combiner 206 and corrector 210 may apply channel rate-of-change (δ₀, δ₁)205 to, at least in part, compensate for distortion in channel 104(FIG. 1) caused by high-Doppler shift, although the scope of theinvention is not limited in this respect. In these embodiments, finalhard-symbol outputs (x₀, x₁) 213 generated by maximum-likelihooddetector 208 are estimates of the transmitted Alamouti-encoded symbols(α₀, α₁) discussed above.

In some embodiments, space-time decoder 200 may also include channelcoefficient estimator 202 to calculate channel coefficients (h₀, h₁) 203based on training signals transmitted through channel 113A (FIG. 1) andchannel 113B (FIG. 1). In some embodiments, channel coefficientestimator 202 may also calculate channel rate-of-change (δ₀, δ₁) 205from two or more sets of channel coefficients 203. In some embodiments,training signals may be transmitted separately (at different times) byeach of transmit antennas 112 (FIG. 1) allowing channel coefficientestimator 202 to separately determine the channel coefficients 203 forchannel 113A (FIG. 1) and channel 113B (FIG. 1).

In some embodiments, corrector 210 generates revised soft-symbol outputs(θ₀, θ₁) 211 and maximum-likelihood detector 208 generates interimhard-symbol outputs 215 iteratively (i.e., one or more times) beforegenerating final hard-symbol outputs (x₀, x₁) 213, although the scope ofthe invention is not limited in this respect. In some embodiments, asingle iteration may be sufficient.

In some embodiments, space-time decoder 200 may also include switchingcircuitry 220 to switch the inputs of maximum-likelihood detector 208from the outputs of combiner 206 to the outputs of the corrector 210.Space-time decoder 200 may also include switching circuitry 222 toswitch the outputs of maximum-likelihood detector 208 to inputs ofcorrector 210 when corrector 210 generates revised soft-symbol outputs211 and maximum-likelihood detector 208 generates interim hard-symboloutputs 215.

In accordance with some embodiments, received symbols (r₀, r₁) 201 maycomprise non-equalized received symbols. In these embodiments, receivedsymbols (r₀, r₁) 201 may be processed by combiner 206 without a priorapplication of channel coefficients. Accordingly, in these embodiments,a channel equalizer is not necessary.

In some embodiments, combiner 206 may generate soft-symbol outputs (ν₀,ν₁) 207 based substantially on the following equations:

$\begin{matrix}{{v_{0} = \frac{( {{R_{1}^{*}h_{1}} + {R_{0}{h_{0}^{*}( {1 + \delta_{0}^{*}} )}}} )}{{h_{0}{h_{0}^{*}( {1 + \delta_{0}^{*}} )}} + {h_{1}{h_{1}^{*}( {1 + \delta_{1}^{*}} )}}}}{v_{1} = \frac{( {{R_{1}^{*}h_{0}} - {R_{0}{h_{1}^{*}( {1 + \delta_{1}^{*}} )}}} )}{{h_{0}{h_{0}^{*}( {1 + \delta_{0}^{*}} )}} + {h_{1}{h_{1}^{*}( {1 + \delta_{1}^{*}} )}}}}} & (7)\end{matrix}$

These equations may compensate, at least in part, for Doppler-induceddistortion. In these equations, r₀ represents a symbol received at afirst instance in time, r₁ represents a symbol received at a secondinstance in time, h₀ represents a channel coefficient for channel 113A(FIG. 1), h₁ represents a channel coefficient for channel 113B (FIG. 1),δ₀ represents a rate-of-change for channel 113A (FIG. 1), δ₁ representsa rate-of-change for the channel 113B (FIG. 1), and * represents thecomplex conjugate. In Equations (7), the denominator is a complexscaling coefficient that may be used to scale the amplitude and phase ofthe received symbols.

In some embodiments, additional processing may be performed. In theseembodiments, the decisions represented by soft-symbol outputs (ν₀, ν₁)207 may be further improved for high-Doppler environments. In theseembodiments, corrector 210 may generate revised soft-symbol outputs (θ₀,θ₁) 211 based substantially on the following equations:

$\begin{matrix}{{\theta_{0} = {\frac{{( {{R_{1}^{*}h_{1}} + {R_{0}h_{0}^{*}}} )( {{h_{0}h_{0}^{*}} + {h_{1}h_{1}^{*}} + {h_{0}h_{0}^{*}\delta_{0}^{*}}} )} - {R_{0}h_{1}h_{1}^{*}h_{0}^{*}\delta_{0}^{*}\delta_{1}^{*}}}{{h_{0}{h_{0}^{*}( {1 + \delta_{0}^{*}} )}} + {h_{1}{h_{1}^{*}( {1 + \delta_{1}^{*}} )}}} + {{\overset{\Cup}{x}}_{1}h_{1}h_{0}^{*}\delta_{0}^{*}}}}{\theta_{1} = {\frac{{( {{{- R_{1}^{*}}h_{0}} + {R_{0}h_{1}^{*}}} )( {{h_{0}h_{0}^{*}} + {h_{1}h_{1}^{*}} + {h_{1}h_{1}^{*}\delta_{1}^{*}}} )} - {R_{0}h_{0}h_{0}^{*}h_{1}^{*}\delta_{0}^{*}\delta_{1}^{*}}}{{h_{0}{h_{0}^{*}( {1 + \delta_{0}^{*}} )}} + {h_{1}{h_{1}^{*}( {1 + \delta_{1}^{*}} )}}} + {{\overset{\Cup}{x}}_{0}h_{0}h_{1}^{*}\delta_{1}^{*}}}}} & (8)\end{matrix}$

In these equations, {hacek over (x)}_(i) represents interim outputs ofmaximum-likelihood detector 208 based on soft-symbol outputs ν_(i).Based on the values of revised soft-symbol outputs (θ₀, θ₁) 211,maximum-likelihood detector 208 may generate final decisions ashard-symbol outputs 213.

In some embodiments, to increase the decoder efficiency, the interimoutputs {hacek over (x)}_(i) and the revised soft-symbol outputs θ^(i)may be recalculated as illustrated by the following equations:

{hacek over (x)}₀←MLDecoding(θ₀)

{hacek over (x)}₁←MLDecoding(θ₁)  (9)

In these embodiments, revised soft decisions illustrated as revisedsoft-symbol outputs θ₀ and θ₁ may be calculated in accordance withEquations (8). This process may be repeated several times, although thescope of the invention is not limited in this respect. In exampleembodiments that use BPSK modulation on a channel with a Rayleighdistribution, for a complex channel rate-of-change (δ_(i)) and a normaldistribution for the real and imaginary parts with variance of about0.25, a significant BER reduction may be achieved over that of standardAlamouti decoding, although the scope of the invention is not limited inthis respect.

In some embodiments that communicate orthogonal frequency divisionmultiplexed (OFDM) signals, receiver 106 may also include Fouriertransform circuitry to generate frequency domain signals correspondingto received symbols 201 from time-domain signals received by antenna 116(FIG. 1). In these embodiments, receiver 106 may also include errorcorrection circuitry, such as a forward error correcting (FEC) decoder,to perform error-correcting decoding on hard-bit outputs and/or soft-bitoutputs, although the scope of the invention is not limited in thisrespect. In some embodiments, hard-bit outputs and soft-bit outputs maybe generated by demodulating hard-symbol outputs 213 and soft-symboloutputs 207, respectively. Receiver 106 (FIG. 1) may also have otherfunctional elements that may be part of its physical layer circuitry togenerate a decoded bit stream corresponding to transmitted symbols notseparately illustrated.

Although space-time decoder 200 is illustrated as having severalseparate functional elements, one or more of the functional elements maybe combined and may be implemented by combinations ofsoftware-configured elements, such as processing elements includingdigital signal processors (DSPs), and/or other hardware elements. Forexample, some elements may comprise one or more microprocessors, DSPs,application specific integrated circuits (ASICs), and combinations ofvarious hardware and logic circuitry for performing at least thefunctions described herein. In some embodiments, the functional elementsof space-time decoder 200 may refer to one or more processes operatingon one or more processing elements.

FIG. 3 is a flow chart of a procedure for decoding signals in a channelwith high-Doppler shift in accordance with some embodiments of thepresent invention. Procedure 300 may be performed by a space-timedecoder, such as space-time decoder 200 (FIG. 1), although other decoderconfigurations may also be suitable. Although procedure 300 isapplicable for decoding signals in a channel with high-Doppler shift,procedure 300 is also applicable for decoding signals with little or noDoppler shift.

In operation 302, channel coefficients (h₀, h₁) are calculated. In someembodiments, channel coefficient estimator 202 (FIG. 1) may calculateone or more sets of channel coefficients from training signals receivedfrom a transmitter, such as transmitter 102 (FIG. 1).

In operation 304, a channel rate-of-change (δ₀, δ₁) is calculated fromchannel coefficients. In some embodiments, channel coefficient estimator202 (FIG. 2) may calculate the channel rate-of change from the channelcoefficients generated in operation 302.

In operation 306, initial soft-symbol outputs (ν₀, ν₁) are generated. Insome embodiments, combiner 206 (FIG. 2) may generate the initialsoft-symbol outputs ν₀ and ν₁ from received symbols r₀ and r₁, thechannel rate-of-change δ₀ and δ₁, and the channel coefficients h₀ andh₁. In some embodiments, the initial soft-symbol outputs ν₀ and ν₁ maybe generated based substantially on Equations (7) discussed above.

In operation 308, initial hard-symbol outputs (x₀, x₁) are generated. Insome embodiments, initial hard-symbol outputs x₀ and x₁ may be generatedby maximum-likelihood detector 208 (FIG. 2) based on initial soft-symboloutputs ν₀ and ν₁) generated in operation 306.

In operation 310, revised soft-symbol outputs (θ₀, θ₁) are generated. Insome embodiments, corrector 210 (FIG. 2) may generate revisedsoft-symbol outputs θ₀ and θ₁ from the initial hard-symbol outputs x₀and x₁, the received symbols r₀ and r₁, the channel rate-of-change δ₀and δ₁, and the channel coefficients h₀ and h₁. In some embodiments,revised soft-symbol outputs θ₀ and θ₁ may be generated based onEquations (8) discussed above.

In operation 312, revised hard-symbol outputs (x₀, x₁) are generated. Insome embodiments, the revised hard-symbol outputs x₀ and x₁ may begenerated by maximum-likelihood detector 208 (FIG. 2) based on therevised soft-symbol outputs θ₀ and θ₁ generated in operation 310.

In operation 314, operations 310 and 312 may be repeated to generaterevised hard-symbol outputs (x₀, x₁). In some embodiments, operation 314is optional. In these embodiments, the revised hard-symbol outputs x₀and x₁ may be generated with a single iteration of operations 310 and312, although the scope of the invention is not limited in this respect.

Upon the completion of operation 312 or 314, final hard-symbol outputsx₀ and x₁ may be provided by space-time decoder 200 (FIG. 2) forsubsequent processing to generate an output-symbol stream that maycorrespond to the symbol stream encoded by transmitter 102 (FIG. 1). Theoutput symbol stream may be demodulated based on the modulation level togenerate an output bit stream which may correspond to the bit streammodulated by transmitter 102 (FIG. 1).

Although the individual operations of procedure 300 are illustrated anddescribed as separate operations, one or more of the individualoperations may be performed concurrently, and nothing requires that theoperations be performed in the order illustrated.

Referring back to FIG. 1, in some embodiments, transmitter 102 andreceiver 106 may communicate OFDM communication signals over amulticarrier communication channel. The multicarrier communicationchannel may be within a predetermined frequency spectrum and maycomprise a plurality of orthogonal subcarriers. In some embodiments, themulticarrier signals may be defined by closely spaced OFDM subcarriers.In some embodiments, transmitter 102 and receiver 106 may communicate inaccordance with a multiple access technique, such as orthogonalfrequency division multiple access (OFDMA), although the scope of theinvention is not limited in this respect. In some other embodiments,transmitter 102 and receiver 106 may communicate using spread-spectrumsignals, although the scope of the invention is not limited in thisrespect.

In some embodiments, transmitter 102 and/or receiver 106 may be part ofa communication station, such as wireless local area network (WLAN)communication station including a Wireless Fidelity (WiFi) communicationstation, an access point (AP) or a mobile station (MS). In some otherembodiments, transmitter 102 and/or receiver 106 may be part of abroadband wireless access (BWA) network communication station, such as aWorldwide Interoperability for Microwave Access (WiMax) communicationstation, although the scope of the invention is not limited in thisrespect.

In some embodiments, transmitter 102 and/or receiver 106 may be part ofa portable wireless communication device, such as a personal digitalassistant (PDA), a laptop or portable computer with wirelesscommunication capability, a web tablet, a wireless telephone, a wirelessheadset, a pager, an instant messaging device, a digital camera, anaccess point, a television, a medical device (e.g., a heart ratemonitor, a blood pressure monitor, etc.), or other device that mayreceive and/or transmit information wirelessly.

In some embodiments, the frequency spectrums for the communicationsignals used by transmitter 102 and receiver 106 may comprise either a 5gigahertz (GHz) frequency spectrum or a 2.4 GHz frequency spectrum. Inthese embodiments, the 5 gigahertz (GHz) frequency spectrum may includefrequencies ranging from approximately 4.9 to 5.9 GHz, and the 2.4 GHzspectrum may include frequencies ranging from approximately 2.3 to 2.5GHz, although the scope of the invention is not limited in this respect,as other frequency spectrums are also equally suitable. In some BWAnetwork embodiments, the frequency spectrum for the communicationsignals may comprise frequencies between 2 and 11 GHz, although thescope of the invention is not limited in this respect.

In some embodiments, transmitter 102 and receiver 106 may communicate inaccordance with specific communication standards, such as the Instituteof Electrical and Electronics Engineers (IEEE) standards including IEEE802.11(a), 802.11(b), 802.11(g), 802.11(h) and/or 802.11(n) standardsand/or proposed specifications for wireless local area networks,although the scope of the invention is not limited in this respect asthey may also be suitable to transmit and/or receive communications inaccordance with other techniques and standards. In some broadbandwireless access network embodiments, transmitter 102 and receiver 106may communicate in accordance with the IEEE 802.16-2004 and the IEEE802.16(e) standards for wireless metropolitan area networks (WMANs)including variations and evolutions thereof, although the scope of theinvention is not limited in this respect as they may also be suitable totransmit and/or receive communications in accordance with othertechniques and standards. For more information with respect to the IEEE802.11 and IEEE 802.16 standards, please refer to “IEEE Standards forInformation Technology—Telecommunications and Information Exchangebetween Systems”—Local Area Networks—Specific Requirements—Part 11“Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY),ISO/IEC 8802-11: 1999”, and Metropolitan Area Networks—SpecificRequirements—Part 16: “Air Interface for Fixed Broadband Wireless AccessSystems,” May 2005 and related amendments/versions. Some embodimentsrelate to the IEEE 802.11e proposed enhancement to the IEEE 802.11 WLANspecification that will include quality of service (QoS) features,including the prioritization of data, voice, and video transmissions.

Transmit antennas 112 and receive antenna 116 may comprise one or moredirectional or omnidirectional antennas, including, for example, dipoleantennas, monopole antennas, patch antennas, loop antennas, microstripantennas or other types of antennas suitable for transmission of RFsignals. In some multiple-input, multiple-output (MIMO) embodiments, twoor more antennas may be used. In some embodiments, instead of two ormore antennas, a single antenna with multiple apertures may be used. Inthese embodiments, each aperture may be considered a separate antenna.

In some other embodiments, transmitter 102 and receiver 106 maycommunicate in accordance with standards such as the Pan-European mobilesystem standard referred to as the Global System for MobileCommunications (GSM). Transmitter 102 and receiver 106 may also operatein accordance with packet radio services such as the General PacketRadio Service (GPRS) packet data communication service. In someembodiments, transmitter 102 and receiver 106 may communicate inaccordance with the Universal Mobile Telephone System (UMTS) for thenext generation of GSM, which may, for example, implement communicationtechniques in accordance with 2.5G and 3G wireless standards (See 3GPPTechnical Specification, Version 3.2.0, March 2000), including the 3GPPlong-term evolution (LTE). In some of these embodiments, transmitter 102and receiver 106 may provide packet data services (PDS) utilizing packetdata protocols (PDP). In some embodiments, transmitter 102 and receiver106 may communicate in accordance with other standards or otherair-interfaces including interfaces compatible with the enhanced datafor GSM evolution (EDGE) standards (see 3GPP Technical Specification,Version 3.2.0, March 2000), although the scope of the invention is notlimited in this respect.

Unless specifically stated otherwise, terms such as processing,computing, calculating, determining, displaying, or the like, may referto an action and/or process of one or more processing or computingsystems or similar devices that may manipulate and transform datarepresented as physical (e.g., electronic) quantities within aprocessing system's registers and memory into other data similarlyrepresented as physical quantities within the processing system'sregisters or memories, or other such information storage, transmission,or display devices. Furthermore, as used herein, a computing deviceincludes one or more processing elements coupled with computer-readablememory that may be volatile or non-volatile memory or a combinationthereof.

Some embodiments of the invention may be implemented in one or acombination of hardware, firmware, and software. Embodiments of theinvention may also be implemented as instructions stored on amachine-readable medium, which may be read and executed by at least oneprocessor to perform the operations described herein. A machine-readablemedium may include any mechanism for storing or transmitting informationin a form readable by a machine (e.g., a computer). For example, amachine-readable medium may include read-only memory (ROM),random-access memory (RAM), magnetic disk storage media, optical storagemedia, flash-memory devices, electrical, optical, acoustical or otherform of propagated signals (e.g., carrier waves, infrared signals,digital signals, etc.), and others.

The Abstract is provided to comply with 37 C.F.R. Section 1.72(b)requiring an abstract that will allow the reader to ascertain the natureand gist of the technical disclosure. It is submitted with theunderstanding that it will not be used to limit or interpret the scopeor meaning of the claims. The following claims are hereby incorporatedinto the detailed description, with each claim standing on its own as aseparate preferred embodiment.

1. A space-time decoder comprising: a combiner to generate soft-symboloutputs from received symbols, a channel rate-of-change and channelcoefficients; and a maximum-likelihood detector to performmaximum-likelihood decoding to generate hard-symbol outputs from thesoft-symbol outputs.
 2. The decoder of claim 1 wherein the hard-symboloutputs are initial hard-symbol outputs, wherein the decoder furthercomprises a corrector for generating revised soft-symbol outputs fromthe initial hard-symbol outputs, the received symbols, the channelrate-of-change and the channel coefficients, and wherein themaximum-likelihood detector generates final hard-symbol outputs byperforming maximum-likelihood decoding on the revised soft-symboloutputs.
 3. The decoder of claim 2 wherein the received symbols compriseAlamouti-encoded symbols transmitted by two transmit antennas, whereinthe combiner and the corrector apply the channel rate-of-change to atleast in part, compensate for distortion in a channel caused by highDoppler shift, and wherein the final hard-symbol outputs generated bythe maximum-likelihood detector are estimates of the transmittedAlamouti-encoded symbols.
 4. The decoder of claim 1 further comprising achannel coefficient estimator to calculate the channel coefficientsbased on training signals transmitted for two or more channels between atransmitter and a receiver, and wherein the channel coefficientestimator further calculates the channel rate-of-change from two or moresets of the channel coefficients.
 5. The decoder of claim 2 wherein thecorrector generates the revised soft-symbol outputs and themaximum-likelihood detector generates interim hard-symbol outputsiteratively prior to generating the final hard-symbol outputs.
 6. Thedecoder of claim 5 further comprising switching circuitry to switchinputs of the maximum-likelihood detector from outputs of the combinerto outputs of the corrector and to switch outputs of themaximum-likelihood detector to inputs of the corrector when thecorrector and the maximum-likelihood detector iteratively generateinterim hard-symbol outputs.
 7. The decoder of claim 3 wherein thereceived symbols comprise non-equalized received symbols.
 8. The decoderof claim 2 wherein the channel rate-of-change comprises a channelrate-of-change for a first channel between a transmitter and a receiverand a channel rate-of-change for a second channel between thetransmitter and the receiver, wherein the channel coefficients comprisea channel coefficient for the first channel and a channel coefficientfor the second channel, wherein the received symbols comprise first andsecond sequentially received symbols, and wherein the combiner generatesa first soft-symbol output based on one plus a complex conjugate of thechannel rate-of-change for the first channel times a complex conjugateof the channel coefficient for the first channel times a first receivedsignal.
 9. The decoder of claim 8 wherein the combiner generates asecond soft-symbol output based on one plus the complex conjugate of thechannel rate-of-change for the second channel times the complexconjugate of the channel coefficient for the second channel times thefirst received signal.
 10. The decoder of claim 2 wherein the receivedsymbols are orthogonal frequency division multiplexed frequency-domainsignals and are generated by performing a Fourier transform on receivedtime-domain signals.
 11. A method for decoding received symbolscomprising: generating soft-symbol outputs from received symbols, achannel rate-of-change and channel coefficients; and performingmaximum-likelihood decoding to generate hard-symbol outputs from thesoft-symbol outputs.
 12. The method of claim 11 wherein the hard-symboloutputs are initial hard-symbol outputs, wherein the method furthercomprises: generating revised soft-symbol outputs from the initialhard-symbol outputs, the received symbols, the channel rate-of-changeand the channel coefficients; and generating final hard-symbol outputsby performing maximum-likelihood decoding on the revised soft-symboloutputs.
 13. The method of claim 12 wherein the received symbolscomprise Alamouti-encoded symbols transmitted by two transmit antennas,wherein the method further comprises applying the channel rate-of-changeto, at least in part, compensate for distortion in a channel caused byhigh Doppler shift, and wherein the final hard-symbol outputs compriseestimates of the transmitted Alamouti-encoded symbols.
 14. The method ofclaim 11 further comprising: calculating the channel coefficients basedon training signals transmitted for two or more channels between atransmitter and a receiver; and calculating the channel rate-of-changefrom two or more sets of the channel coefficients.
 15. The method ofclaim 12 further comprising generating the revised soft-symbol outputsand interim hard-symbol outputs iteratively prior to generating thefinal hard-symbol outputs.
 16. The method of claim 15 wherein a combinergenerates the soft-symbol outputs, a maximum-likelihood detectorgenerates the interim and the final hard-symbol outputs, and a correctorgenerates the revised soft-symbol outputs, and wherein the methodfurther comprises: switching inputs of the maximum-likelihood detectorfrom outputs of the combiner to outputs of the corrector; and switchingoutputs of the maximum-likelihood detector to inputs of the correctorwhen the corrector and the maximum-likelihood detector iterativelygenerates the interim hard-symbol outputs.
 17. The method of claim 13wherein the received symbols comprise non-equalized received symbols.18. The method of claim 12 wherein the received symbols are orthogonalfrequency division multiplexed (OFDM) frequency-domain signals and aregenerated by performing a Fourier transform on received time-domainsignals.
 19. A receiver comprising: a space-time decoder comprising acombiner and a maximum-likelihood detector, the combiner to generatesoft-symbol outputs from received symbols, a channel rate-of-change andchannel coefficients, the maximum-likelihood detector to performmaximum-likelihood decoding to generate hard-symbol outputs from thesoft-symbol outputs; and a substantially omnidirectional antenna toreceive signals comprising the received symbols.
 20. The receiver ofclaim 19 wherein the hard-symbol outputs are initial hard-symboloutputs, wherein the space-time decoder further comprises a correctorfor generating revised soft-symbol outputs from the initial hard-symboloutputs, the received symbols, the channel rate-of-change and thechannel coefficients, and wherein the maximum-likelihood detectorgenerates final hard-symbol outputs by performing maximum-likelihooddecoding on the revised soft-symbol outputs.
 21. The receiver of claim20 wherein the received symbols comprise Alamouti-encoded symbolstransmitted by two transmit antennas, wherein the combiner and thecorrector apply the channel rate-of-change to at least in part,compensate for distortion in a channel caused by high Doppler shift, andwherein the final hard-symbol outputs generated by themaximum-likelihood detector are estimates of the transmittedAlamouti-encoded symbols.
 22. The receiver of claim 19 furthercomprising a channel coefficient estimator to calculate the channelcoefficients based on training signals transmitted for two or morechannels between a transmitter and the receiver, and wherein the channelcoefficient estimator further calculates the channel rate-of-change fromtwo or more sets of the channel coefficients.
 23. The receiver of claim20 wherein the corrector generates the revised soft-symbol outputs andthe maximum-likelihood detector generates interim hard-symbol outputsiteratively prior to generating the final hard-symbol outputs.